1. Field of the Invention
The present invention relates to a flash monitor for determining the excitation of a flash tube in photography and, more particularly, to a flash triggering system for synchronizing "fill-in" strobe flashes with a through-the-lens metering master strobe flash.
2. Description of Related Art
A recurring problem in the art of strobe flashes for photographic purposes is the problem of ensuring that additional strobes, other than the master strobe directly attached to the camera, fire in concert with the master strobe. Sometimes the additional or "slave" strobes are necessary to provide primary illumination to all or part of the photographed scene. More often the slave flashes are used for "fill-in" to lighten shadows in the scene and thereby avoid the harsh shadows caused by unidirectional illumination.
Although the slave flashes could be directly attached by cables to the master flash, it is more desirable to use some type of remote flash triggering system to trigger the slave flashes, thereby avoiding the need for cumbersome cables. Essentially, the flash triggering system detects the occurrence of the master flash and then triggers the slave flashes by means of infrared signals, radio waves, or other wireless methods.
Major problems with flash triggering systems are false alarms where the slave flashes are triggered without the master flash first firing, and missed alarms where the slave flashes are not triggered despite the firing of the master strobe flash. The photographic flash monitor triggering art is generally cognizant of flash triggering systems which utilize photosensors to detect light from the master flash and thereby trigger the slave flashes. Unfortunately, it is often difficult to properly set the sensitivity of photosensor-based devices. Insufficient sensitivity of the detector will result in missed alarms, while excess sensitivity will result in false alarms.
Although flash triggering systems are available, the art is still seeking an inexpensive, convenient, easily-mounted flash triggering system capable of reliably distinguishing between a true inductive flash tube discharge pulse which is associated with the proper firing of a flash tube and a high-voltage trigger pulse not followed by a discharge pulse, such a trigger pulse being associated with a flash tube misfire. Such a device can be used to reliably trigger remote slave flash units.
An additional problem has been posed by the growing popularity of Through-The-Lens (TTL) flash metering. With more traditional strobes the exposure (i.e., camera lens opening) is set according to distance of the flash from the object or, more modernly, by having a photosensor on the strobe that measures the light reflected from a scene to be photographed and cuts off the flash when sufficient light has been reflected from the scene. TTL metering goes one step farther and measures the reflected light at a film plane within the camera.
In many TTL systems a small photosensor inside the camera actually pops up in front of the film plane to obtain an accurate reading. Because the sensor would actually cast a shadow on the film, these systems work by having the master strobe produce a "preflash" for the sensor to read. The system calculates the correct exposure and then produces a "true" flash for the actual exposure with the photosensor out of the film plane. Such double flashing causes all sorts of problems for the slave strobes. It is important to trigger them for fill-in purposes when the camera shutter is open, but they should not interfere with the main flash of the TTL or with the sensing of the main exposure. Furthermore, the actual main TTL flash produces significant radio and optical interference that can confound the control signals from a flash triggering systems to remote slave flashes.